Controlling nanofiber sheet width

ABSTRACT

Techniques are described for controlling widths of nanofiber sheets drawn from a nanofiber forest. Nanofiber sheet width can be controlled by dividing or sectioning the nanofiber sheet in its as-drawn state into sub-sheets as the sheet is being drawn. A width of a sub-sheet can be controlled or selected so as to contain regions of uniform nanofiber density within a sub-sheet (thereby improving nanofiber yarn consistency) or to isolate an inhomogeneity (whether a discontinuity is the sheet (e.g., a tear) or a variation in density) within a sub-sheet. Techniques for dividing a nanofiber sheet into sub-sheets includes mechanical, corona, and electrical arc techniques.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Divisional of U.S. patent application Ser. No.16/114,464, filed on Aug. 28, 2018, which claims the benefit of U.S.Provisional Pat. Appl. Nos. 62/579,264, filed Oct. 31, 2017 and62/561,779 filed Sep. 22, 2017. The disclosure of each of thesedocuments, including the specification, drawings, and claims, isincorporated herein by reference in its entirety.

TECHNICAL FIELD

The present disclosure relates generally to nanofibers. Specifically,the present disclosure relates to controlling nanofiber sheet width.

BACKGROUND

Nanofiber forests, composed of both single wall and multiwallednanotubes, can be drawn into nanofiber ribbons or sheets. In itspre-drawn state, the nanofiber forest comprises a layer (or severalstacked layers) of nanofibers that are parallel to one another andperpendicular to a surface of a growth substrate. When drawn into ananofiber sheet, the orientation of the nanofibers changes fromperpendicular to parallel relative to the surface of the growthsubstrate. The nanotubes in the drawn nanofiber sheet connect to oneanother in an end-to-end configuration to form a continuous sheet inwhich a longitudinal axis of the nanofibers is parallel to a plane ofthe sheet (i.e., parallel to both of the first and second major surfacesof the nanofiber sheet). The nanofiber sheet can be treated in any of avariety of ways, including spinning the nanofiber sheet into a nanofiberyarn.

SUMMARY

Example 1 is a method for dividing a nanofiber sheet, the methodcomprising: providing a nanofiber sheet comprising a plurality ofnanofiber oriented parallel to a direction in which the nanofiber sheetis drawn, the nanofiber sheet having a first major surface and a secondmajor surface opposite the first major surface; positioning at least oneelectrode proximate to, and not in direct contact with, at least one ofthe major surfaces of the nanofiber sheet; applying a voltage to the atleast one electrode proximate to the at least one major surface of thenanofiber sheet; generating an electrical discharge at the at least oneelectrode from the applied voltage;

and using the electrical discharge, dividing the nanofiber sheet intotwo or more sub-sheets by removing a portion of the nanofiber sheet.

Example 2 includes the subject matter of Example 1, wherein positioningthe at least one electrode proximate to, and not in direct contact with,a major surface of the nanofiber sheet includes generating a corona thatremoves the portion of the nanofiber sheet.

Example 3 includes the subject matter of either of Examples 1 or 2,further comprising increasing a size of the portion of the nanofibersheet that is removed by increasing a magnitude of the applied voltage.

Example 4 includes the subject matter of any of the preceding Examples,wherein positioning the at least one electrode further comprises:placing a first arc discharge electrode proximate to, and not in directcontact with, a first major surface of the nanofiber sheet; placing asecond arc discharge electrode proximate to, and not in direct contactwith, a second major surface of the nanofiber sheet, wherein the voltageis applied to at least one of the first arc discharge electrode and thesecond arc discharge electrode; and responsive to the applied voltage,causing an electrical arc to flow between the first arc dischargeelectrode and the second arc discharge electrode.

Example 5 includes the subject matter of any of the preceding Examples,further comprising: identifying an inhomogeneity in the nanofiber sheet;and positioning the at least one electrode proximate to, and not indirect contact with, the inhomogeneity in the nanofiber sheet.

Example 6 includes the subject matter of Example 5, wherein dividing thenanofiber sheet comprises dividing the nanofiber sheet into at least afirst sub-sheet and a second sub-sheet, wherein the inhomogeneity isdisposed entirely within the first sub-sheet.

Example 7 includes the subject matter of either of Examples 5 or 6,wherein the inhomogeneity is a tear in the nanofiber sheet.

Example 8 includes the subject matter of any of Examples 5-7, whereinthe inhomogeneity is a variation in a number of nanofibers per unitvolume of the nanofiber sheet.

Example 9 includes the subject matter of any of the preceding Examples,further comprising: spinning a nanofiber yarn from at least one of twoor more sub-sheets; monitoring one or more of a diameter and anelectrical property of the nanofiber yarn during the spinning; andre-positioning the at least one electrode proximate to, and not indirect contact with, the nanofiber sheet in response to the monitoreddiameter or electrical properties of the nanofiber yarn, there-positioning changing a width of the two or more sub-sheets byrelocating the electrical discharge.

Example 10 includes the subject matter of Example 9, whereinre-positioning the at least one electrode reduces a variation indiameter of the nanofiber yarn to less than +/−5% over a length of 1 mm.

Example 11 includes the subject matter of any of the preceding Examples,wherein at least one of the two or more sub-sheets is less than 5 μm inwidth.

Example 12 includes the subject matter of Example 11, further comprisingspinning a nanofiber yarn less than 1 μm in diameter from the sub-sheetless than 5 μm in width.

Example 13 includes the subject matter of any of Examples 11-12, furthercomprising simultaneously spinning a separate nanofiber yarn from eachof the at least two or more sub-sheets.

Example 14 includes the subject matter of any of the preceding Examples,wherein the nanofiber sheet is a carbon nanofiber sheet and whereincarbon nanofibers of the carbon nanofiber sheet are multiwalled carbonnanofibers having a diameter of less than 100 nm.

Example 15 includes the subject matter of any of the preceding Examples,wherein positioning the at least one electrode further comprises placingthe at least one electrode proximate to, and not in direct contact with,a major surface of the nanofiber sheet, wherein the applied voltagegenerates an electrical arc between the at least one electrode and thenanofiber sheet, the electrical arc removing the portion of thenanofiber sheet.

Example 16 is a method for dividing a nanofiber sheet, the methodcomprising configuring a structure to have at least one salient featurecomprising an edge, and at least one reverse salient feature; contactingthe nanofiber sheet with an edge of the at least one salient feature;and drawing the nanofiber sheet past the at least one salient featurewhile maintaining contact therebetween, the drawing causing thenanofiber sheet to be dividing into sub-sheets.

Example 17 includes the subject matter of Example 16, wherein a width ofthe at least one salient feature is at least 1 μm.

Example 18 includes the subject matter of either of Examples 16 or 17,wherein a height of the at least one salient feature is at least 1 μm.

Example 19 includes the subject matter of any of Examples 16-18, furthercomprising identifying an inhomogeneity in the nanofiber sheet, andwherein dividing the nanofiber sheet comprises dividing the nanofibersheet into at least a first sub-sheet and a second sub-sheet, theinhomogeneity disposed entirely within the first sub-sheet.

Example 20 includes the subject matter of Example 19, wherein theinhomogeneity is a tear in the nanofiber sheet or a variation in anumber of nanofibers per unit volume of the nanofiber sheet.

Example 21 includes the subject matter of any of examples 19-20, whereinat least one of the first or second sub-sheets is less than 5 μm inwidth.

Example 22 includes the subject matter of Example 21, further comprisingspinning a nanofiber yarn less than 1 μm in diameter from the sub-sheetless than 5 μm in width.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an example forest of nanofibers on a substrate, in anembodiment.

FIG. 2 illustrates an example reactor used for growing nanofibers, in anembodiment.

FIG. 3 is an illustration of a nanofiber sheet that identities relativedimensions of the sheet and schematically illustrates nanofibers withinthe sheet aligned end-to-end in a plane parallel to a surface of thesheet, in an embodiment.

FIG. 4 is an image of a nanofiber sheet being laterally drawn from ananofiber forest, the nanofibers aligning from end-to-end asschematically shown in FIG. 3.

FIGS. 5A to 5C illustrate nanofiber sheets being drawn from a nanofiberforest in which non-spinnable particles that are present cause defectsin the nanofiber sheets, in embodiments.

FIG. 6A illustrates a one-electrode electrical discharge system fordividing a nanofiber sheet into sub-sheets using a contactless dividingtechnique, in an embodiment.

FIG. 6B illustrates a two-electrode electrical discharge system fordividing a nanofiber sheet into sub-sheets using a contactless dividingtechnique, in an embodiment.

FIGS. 7A and 7B illustrate plan views of a corona discharge systemdividing a nanofiber sheet into sub-sheets, without physical contactbetween an electrode and the nanofiber sheet, in an embodiment.

FIG. 8 illustrates altering sizes of electrical discharges to controlsub-sheet width, in an embodiment.

FIGS. 9A and 9B are a side view and a plan view, respectively, of amechanical system for dividing a nanofiber sheet into sub-sheets, in anembodiment.

FIG. 10 is a method flow diagram of an example method for dividing ananofiber sheet into at least two sub-sheets, in an embodiment.

The figures depict various embodiments of the present disclosure forpurposes of illustration only. Numerous variations, configurations, andother embodiments will be apparent from the following detaileddiscussion.

DETAILED DESCRIPTION Overview

Nanofiber sheets drawn from a nanofiber forest can be inhomogeneous forany of a variety of reasons. For example, due to variations in thenumber of nanofibers per unit area of growth substrate, a density of ananofiber sheet (both in terms of nanofibers per unit volume and/or massper unit volume) can vary across one or both a width and a length of thesheet. Because a diameter of a nanofiber yarn spun from a nanofibersheet is a function of nanofiber density within the sheet, these densityvariations within a sheet can cause diameter variations within ananofiber yarn spun from the sheet. In some examples, the diametervariation can be +/−one micron (1 μm) or more. A variability in diameterof one or more microns for a nanofiber yarn having a diameter on theorder of tens of microns is a significant deviation, Diameter variationof a yarn can cause variations in mechanical properties (e.g., ultimatetensile strength), and electrical properties (e.g., conductivity), amongother properties. These types of variations can make nanofiber yarnsless appealing as a material for use in applications that strive forconsistent or predictable performance.

Controlling nanofiber density on a growth substrate during nanofiberforest growth can be difficult. Similarly, controlling a density ofnanofibers drawn into a nanofiber yarn during the drawing process canalso be difficult.

Complicating the control of nanofiber density is the presence of defectswithin the nanofiber forest. One type of defect is an impurity that isnot drawn into the nanofiber sheet or spun into the nanofiber yarn. Forexample, a particle that is not drawn into a yarn during the drawingprocess (e.g., a particle that does not have a longitudinal axis, butrather is more sphere-like or irregularly shaped or a particle attachedto the growth substrate) can disrupt the drawing of a sheet by causing asheet to tear or otherwise fail during drawing.

While laser-assisted cutting or blade-based cutting of nanofiber sheetsinto sub-sheets can be performed, neither of these techniques areflexible and convenient enough to maneuver around defects or be adjustedduring the drawing process without damaging the sheet. Nor arelaser-assisted cutting or blade-based cutting generally used to maintaina consistent density of a drawn sheet. Thus, the applicability of thesetechniques in maintaining uniformity of the nanofiber sheet so as toproduce a nanofiber yarn with a uniform diameter (e.g., a diameter thatvaries less than +/−5% from a mean value along a length of the nanofiberyarn) is limited.

Thus, in accordance with an embodiment of the present disclosure,techniques are described for controlling widths of nanofiber sheetsdrawn from a nanofiber forest. In examples, width of a drawn nanofibersheet can be controlled by dividing or sectioning the nanofiber sheet inits as-drawn state into sub-sheets as the sheet is being drawn. A widthof a sub-sheet can be controlled or selected so as to contain regions ofuniform nanofiber density within a sub-sheet (thereby improvingnanofiber yarn consistency) or to isolate an inhomogeneity (whether adiscontinuity is the sheet (e.g., a tear) or a variation in density)within a sub-sheet. By isolating the inhomogeneity in a sub-sheet, theremaining, uniform, sheets can be spun into yarns that have a consistentdiameter. This is more likely to produce a nanofiber yarn havingconsistent mechanical and electrical properties along its entire length.This consistency in diameter, physical/mechanical properties (e.g.,density, tensile strength), and electrical properties makes nanofiberyarns more easily integrated into applications that require consistentand/or predictable properties. In some embodiments, in situ monitoringof nanofiber yarn and/or nanofiber sheet physical properties (e.g., yarndiameter, yarn density, sheet density, sheet width) can be used toposition electrodes that generate an electrical arc and/or corona usedto divide the nanofiber sheet. By moving the arc(s) and/or corona(s) soas to dynamically change a width of a sub-sheet, a diameter and/ordensity of a resulting nanofiber yarn can be more consistent (e.g., to avariation in either diameter and/or density of less than 10% (+/−5%)over a length of a yarn). Furthermore, some of the embodiment techniquesdescribed herein do not require contact between an electrode and thenanofiber sheet that is being divided into sub-sheets. Rather, thedividing is accomplished via an electrical arc and/or corona disposedaround one or more electrodes and that bridges a physical separationbetween a surface of the electrode and the nanofiber sheet beingdivided. Avoiding direct physical contact between an electrode and thenanofiber sheet has at least two benefits. First, movement of anelectrode in response a feedback system used to maintain and/or improveconsistency of a nanofiber yarn produced from the nanofiber sub-sheetcan be faster because there is no direct physical contact between anelectrode and a sheet and thus no friction to overcome and no risk oftearing the nanofiber sheet. Second, the power supplied to the electrodedoes not have “spikes” (i.e., temporary surges in power), but rather ismore uniform over time. This reduces power consumption and reduces therisk of unintentional scorching and/or damage to the nanofiber sheetcaused by power surges.

Prior to describing these techniques, a. description of nanofiber forestgrowth, and nanofiber sheet drawing for context.

Nanofiber Forests

As used herein, the term “nanofiber” means a fiber having a diameterless than 1 μm. While the embodiments herein are primarily described asfabricated from carbon nanotubes, it will be appreciated that othercarbon allotropes, whether graphene, micron or nano-scale graphitefibers and/or plates, and even other compositions of nano-scale fiberssuch as boron nitride may be densified using the techniques describedbelow. As used herein, the terms “nanofiber” and “carbon nanotube”encompass both single walled carbon nanotubes and/or multi-walled carbonnanotubes in which carbon atoms are linked together to form acylindrical structure. In some embodiments, carbon nanotubes asreferenced herein have between 4 and 10 walls. As used herein, a“nanofiber sheet” or simply “sheet” refers to a sheet of nanofibersaligned via a drawing process (as described in PCT Publication No. WO2007/015710, and incorporated by reference herein in its entirety) sothat a longitudinal axis of a nanofiber of the sheet is parallel to amajor surface of the sheet, rather than perpendicular to the majorsurface of the sheet (i.e., in the as-deposited form of the sheet, oftenreferred to as a “forest”). This is illustrated and shown in FIGS. 3 and4, respectively.

The dimensions of carbon nanotubes can vary greatly depending onproduction methods used. For example, the diameter of a carbon nanotubemay be from 0.4 nm to 100 nm and its length may range from 10 μm togreater than 55.5 cm. Carbon nanotubes are also capable of having veryhigh aspect ratios (ratio of length to diameter) with some as high as132,000,000:1 or more. Given the wide range of dimensionalpossibilities, the properties of carbon nanotubes are highly adjustable,or “tunable.” While many intriguing properties of carbon nanotubes havebeen identified, harnessing the properties of carbon nanotubes inpractical applications requires scalable and controllable productionmethods that allow the features of the carbon nanotubes to be maintainedor enhanced.

Due to their unique structure, carbon nanotubes possess particularmechanical, electrical, chemical, thermal and optical properties thatmake them well-suited for certain applications. In particular, carbonnanotubes exhibit superior electrical conductivity, high mechanicalstrength, good thermal stability and are also hydrophobic, in additionto these properties, carbon nanotubes may also exhibit useful opticalproperties. For example, carbon nanotubes may be used in light-emittingdiodes (LEDs) and photo-detectors to emit or detect light at narrowlyselected wavelengths. Carbon nanotubes may also prove useful for photontransport and/or phonon transport.

In accordance with various embodiments of the subject disclosure,nanofibers (including but not limited to carbon nanotubes) can bearranged in various configurations, including in a configurationreferred to herein as a “forest.” As used herein, a “forest” ofnanofibers or carbon nanotubes refers to an array of nanofibers havingapproximately equivalent dimensions that are arranged substantiallyparallel to one another on a substrate. FIG. 1 shows an example forestof nanofibers on a substrate. The substrate may be any shape but in someembodiments the substrate has a planar surface on which the forest isassembled. As can be seen in FIG. 1, the nanofibers in the forest may beapproximately equal in height and/or diameter.

Nanofiber forests as disclosed herein may be relatively dense.Specifically, the disclosed nanofiber forests may have a density of atleast 1 billion nanofibers/cm². In some specific embodiments, ananofiber forest as described herein may have a density of between 10billion/cm² and 30 billion/cm². In other examples, the nanofiber forestas described herein may have a density in the range of 90 billionnanofibers/cm². The forest may include areas of high density or lowdensity and specific areas may be void of nanofibers. The nanofiberswithin a forest may also exhibit inter-fiber connectivity. For example,neighboring nanofibers within a nanofiber forest may be attracted to oneanother by van der Waals forces. Regardless, a density of nanofiberswithin a forest can be increased by applying techniques describedherein.

Methods of fabricating a nanofiber forest are described in, for example,PCT No. WO2007/015710, which is incorporated herein by reference in itsentirety.

Various methods can be used to produce nanofiber precursor forests. Forexample, in some embodiments nanofibers may be grown in ahigh-temperature furnace, schematically illustrated in FIG. 2. In someembodiments, catalyst may be deposited on a substrate, placed in areactor and then may be exposed to a fuel compound that is supplied tothe reactor. Substrates can withstand temperatures of greater than 800°C. or even 1000° C. and may be inert materials. The substrate maycomprise stainless steel or aluminum disposed on an underlying silicon(Si) wafer, although other ceramic substrates may be used in place ofthe Si wafer (e.g., alumina, zirconia, SiO₂, glass ceramics). Inexamples where the nanofibers of the precursor forest are carbonnanotubes, carbon-based compounds, such as acetylene may be used as fuelcompounds. After being introduced to the reactor, the fuel compound(s)may then begin to accumulate on the catalyst and may assemble by growingupward from the substrate to form a forest of nanofibers. The reactoralso may include a gas inlet where fuel compound(s) and carrier gassesmay be supplied to the reactor and a gas outlet where expended fuelcompounds and carrier gases may be released from the reactor. Examplesof carrier gases include hydrogen, argon, and helium. These gases, inparticular hydrogen, may also be introduced to the reactor to facilitategrowth of the nanofiber forest. Additionally, dopants to be incorporatedin the nanofibers may be added to the gas stream.

In a process used to fabricate a multilayered nanofiber forest, onenanofiber forest is formed on a substrate followed by the growth of asecond nanofiber forest in contact with the first nanofiber forest.Multi-layered nanofiber forests can be formed by numerous suitablemethods, such as by forming a first nanofiber forest on the substrate,depositing catalyst on the first nanofiber forest and then introducingadditional fuel compound to the reactor to encourage growth of a secondnanofiber forest from the catalyst positioned on the first nanofiberforest. Depending on the growth methodology applied, the type ofcatalyst, and the location of the catalyst, the second nanofiber layermay either grow on top of the first nanofiber layer or, after refreshingthe catalyst, for example with hydrogen gas, grow directly on thesubstrate thus growing under the first nanofiber layer. Regardless, thesecond nanofiber forest can be aligned approximately end-to-end with thenanofibers of the first nanofiber forest although there is a readilydetectable interface between the first and second forest. Multi-layerednanofiber forests may include any number of forests. For example, amulti-layered precursor forest may include two, three, four, five ormore forests.

Nanofiber Sheets

In addition to arrangement in a forest configuration, the nanofibers ofthe subject application may also be arranged in a sheet configuration.As used herein, the term “nanofiber sheet,” “nanotube sheet,” or simply“sheet” refers to an arrangement of nanofibers where the nanofibers arealigned end to end in a plane. An illustration of an example nanofibersheet is shown in FIG. 3 with labels of the dimensions. In someembodiments, the sheet has a length and/or width that is more than 100times greater than the thickness of the sheet. In some embodiments, thelength, width or both, are more than 10³, 10⁶ or 10⁹ times greater thanthe average thickness of the sheet. A nanofiber sheet can have athickness of, for example, between approximately 5 nm and 30 μm and anylength and width that are suitable for the intended application. In someembodiments, a nanofiber sheet may have a length of between 1 cm and 10meters and a width between 1 cm and 1 meter. These lengths are providedmerely for illustration. The length and width of a nanofiber sheet areconstrained by the configuration of the manufacturing equipment and notby the physical or chemical properties of any of the nanotubes, forest,or nanofiber sheet. For example, continuous processes can produce sheetsof any length. These sheets can be wound onto a roll as they areproduced.

As can be seen in FIG. 3, the axis in which the nanofibers are alignedend-to end is referred to as the direction of nanofiber alignment. Insome embodiments, the direction of nanofiber alignment may be continuousthroughout an entire nanofiber sheet. Nanofibers are not necessarilyperfectly parallel to each other and it is understood that the directionof nanofiber alignment is an average or general measure of the directionof alignment of the nanofibers.

Nanofiber sheets may be assembled using any type of suitable processcapable of producing the sheet. In some example embodiments, nanofibersheets may be drawn from a nanofiber forest. An example of a nanofibersheet being drawn from a nanofiber forest is shown in FIG. 4

As can be seen in FIG. 4, the nanofibers may be drawn laterally from theforest and then align end-to-end to form a nanofiber sheet. Inembodiments where a nanofiber sheet is drawn from a nanofiber forest,the dimensions of the forest may be controlled to form a nanofiber sheethaving particular dimensions. For example, the width of the nanofibersheet may be approximately equal to the width of the nanofiber forestfrom which the sheet was drawn. Additionally, the length of the sheetcan be controlled, for example, by concluding the draw process when thedesired sheet length has been achieved.

Nanofiber sheets have many properties that can be exploited for variousapplications. For example, nanofiber sheets may have tunable opacity,high mechanical strength and flexibility, thermal and electricalconductivity, and may also exhibit hydrophobicity. Given the high degreeof alignment of the nanofibers within a sheet, a nanofiber sheet may beextremely thin. In some examples, a nanofiber sheet is on the order ofapproximately 10 nm thick (as measured within normal measurementtolerances), rendering it nearly two-dimensional. In other examples, thethickness of a nanofiber sheet can be as high as 200 nm or 300 nm. Assuch, nanofiber sheets may add minimal additional thickness to acomponent.

As with nanofiber forests, the nanofibers in a nanofibers sheet may befunctionalized by a treatment agent by adding chemical groups orelements to a surface of the nanofibers of the sheet and that provide adifferent chemical activity than the nanofibers alone. Functionalizationof a nanofiber sheet can be performed on previously functionalizednanofibers or can be performed on previously unfunctionalizednanofibers. Functionalization can be performed using any of thetechniques described herein including, but not limited to CVD, andvarious doping techniques.

Nanofiber sheets, as drawn from a nanofiber forest, may also have highpurity, wherein more than 90%, more than 95% or more than 99% of theweight percent of the nanofiber sheet is attributable to nanofibers, insome instances. Similarly, the nanofiber sheet may comprise more than90%, more than 95%, more than 99% or more than 99.9% by weight ofcarbon.

Defects in Nanofiber Sheets

As indicated above, variations in density within a nanofiber sheet orinclusion of non-spinnable particles can be problematic when spinning ananofiber sheet into a nanofiber yarn. FIGS. 5A-5C illustrate oneexample of a challenge posed by the inclusion of a non-spinnableparticle when drawing a nanofiber forest.

FIG. 5A is a plan view illustration 500 of three nanofiber sheets beingdrawn from a patterned nanofiber forest disposed on a substrate. Whileembodiments described herein are applicable to patterned andnon-patterned forests, the illustration 500 employs a patternedsubstrate so as to simultaneously illustrate two different types ofdefects in nanofiber sheets caused by the presence of non-spinnableparticles. Examples of patterned forests and techniques for patterningforests are described in PCT Publication No. WO 2007/015710, which isincorporated by reference herein in its entirety.

The illustration 500 includes a nanofiber forest 504, a growth substrate508, nanofiber sheets 512A, 512B, and 512C, and particles 524A, 524B.

The nanofiber forest 504 is shown in its as-grown state on the growthsubstrate 508. The example nanofiber forest 504 and the example growthsubstrate 508 have been described above and need no further explanation.It will be appreciated that in some examples the substrate 508 mayactually be a secondary substrate, such as an adhesive substrate, andthe nanofiber forest 504 may have been processed so as to align thenanofibers in a common direction relative to the secondary substrate,such as those described in PCT Application No. PCT/US2017/036687, whichis incorporated by reference herein in its entirety.

As shown, the patterned nanofiber forest 504 is drawn into threenanofiber sheets 512A, 512B, 512C. The processes for drawing a nanofiberforest into a nanofiber sheet (or sheets) are described above, and shownin FIGS. 3 and 4. Nanofiber sheets 512A and 512B are separated by a gap516B. Nanofiber sheets 512B and 512C are separated by a gap 516A. Boththe gaps 516A and 516B can be produced by patterning a substrate orpatterning an arrangement of catalyst on the substrate, in examples.

As the nanofiber sheets 512A, 512B, 512C are drawn from the forest 504,a forest-sheet interface 520 gradually moves toward an opposing edge(not shown) of the substrate in a direction opposite the sheet drawingdirection. Both the movement of the forest-sheet interface 520 and thesheet drawing direction are indicated by arrows in FIG. 5A.

In the illustration 500, the particles 524A, 524B are disposed withinthe nanofiber forest 504. In this example, the particles 524A, 524B arenot only within the nanofiber forest 504, but also fixed relative to thesubstrate 508. The effect of the particles 524A, 524B is to disrupt thealignment and ordering of nanofibers within the drawn nanofiber sheet(e.g., any one or more of sheets 512A, 512B, 512C). This disruption canbe due to the size and/or shape of the particles 524A, 524B. Forexample, spherical (or irregular, spheroidal structures) can causedefects that tear a nanofiber sheet as the sheet is being drawn. Inanother example, large structures, regardless of the shape, can disruptthe organization of a large number of nanofibers. Other mechanisms forcausing a defect in a nanofiber sheet will be appreciated in light ofthe present disclosure.

Particles that can cause a persistent defect within a nanofiber sheet,in examples, can have a least one dimension that is larger than across-sectional thickness of the nanofiber forest. Examples of theseparticles can include foreign objects disposed on the growth substrateor even agglomerations of nanofibers that are not spinnable. Forexample, some nanofibers may grow in a disordered group (i.e., not growin a single layer or not grow parallel to one another). The disorderedorientation of the individual nanofibers within the group then acts as aparticle that can disrupt the drawing of the nanofibers into thenanofiber sheet.

While some nanofiber forests produce sheets that are thick enough andwith sufficient density that the defect, often a tear in the sheet, canbe healed with sufficient drawing (that is, the nanofiber sheet regainsits continuity upon sufficient drawing), other sheets lack this abilityto recover. For example, some nanofiber sheets lack enough nanofibers(whether due to low sheet thickness or low density) sufficient to allowthe portions of the sheet separated by a tear to re-form. Thus, thesesheets cannot “heal” the tear. in other examples, the defect caused bythe particle is large enough so that a sheet, regardless of thenanofiber density within the sheet, is unable to re-form. Much like avariation in density of a continuous nanofiber sheet, the variationcaused by a discontinuity (e.g., a tear) in a nanofiber sheet causesvariation in the diameter and/or properties of a nanofiber yarn.

FIG. 5B continues the example of illustration 500, in which thenanofiber sheets 512A, 512B, 512C have been drawn to consume more of thenanofiber forest 504, thus moving the forest-sheet interface 520 in thedirection indicated and opposite the direction in which the sheets 512A,512B, 512C are drawn. As shown, the forest-sheet interface 520 isadjacent to the particles 524A, 524B. However, the forest has not yetbeen drawn past the particles 524A, 524B.

FIG. 5C illustrates two different types of defects caused by theparticles 524A, 524B. In this example, the forest-sheet interface 520has moved past the particles 524A, 524B in a direction opposite thedirection in which the nanofiber sheets 512A, 512B, 512C are drawn. Assuch, the particle 524A causes a tear 528A to form in the nanofibersheet 512C. The particle 524B causes the sheet 512A to break entirely(indicated by arrow 528B). These defects are consistent with thosedescribed above. For example, particle 524A is either large enough,shaped irregularly enough, or both, to cause a tear in the nanofibersheet 512C. At the same time, the nanofiber sheet 512C is dense enough,wide enough, or both, that the nanofiber sheet 512C can re-form, thus“healing” some of the tear 528A.

The nanofiber sheet 512A however lacks the capability of the nanofibersheet 512C. As is shown in FIG. 5C, a width of the nanofiber sheet 512Ais less than a corresponding dimension of the irregularly shapedparticle 524B. For this reason, when the portion of the forest-sheetinterface 520 corresponding to the nanofiber sheet 512A encounters theparticle 524B, the nanofiber sheet 512A breaks. The discontinuitybetween the nanofiber sheet 512A and its corresponding portion ofnanofiber forest on an opposite side of the particle 524B is indicatedas discontinuity 528B.

As described above, both the fracture of a nanofiber sheet (e.g.,nanofiber sheet 512A) and formation of a tear in a nanofiber sheet(e.g., nanofiber sheet 512C), among other defects not described butapparent to those skilled in the art, affect the consistency ofnanofiber yarns spun from the nanofiber forest.

Controlling Nanofiber Sheet Width

To overcome the challenges posed by inhomogeneities in nanofiber forestsand nanofiber sheets, including those described above, embodimentsdescribed herein include various mechanisms for dividing a nanofibersheet in its as-drawn form into any number of sub-sheets. In examples,the embodiments described herein can be used to maintain a uniformdensity of nanofibers within a sub-sheet, confine a defect to within onesub-sheet so that the remaining sub-sheets can maintain a level ofhomogeneity that can produce a consistent nanofiber yarn (e.g. having aconsistent and uniform diameter (+/−5% from a mean value as measuredalong a length of the yarn) and/or electrical properties), among otherembodiments. In some examples, systems used to divide a nanofiber sheetinto sub-sheets can be selectively configured so that any number ofnanofiber sub-sheets of any width can be fabricated.

Techniques of the present disclosure for dividing a nanofiber sheetinclude both electrical and mechanical techniques. Electrical techniquesinclude using electrical current (whether alternating current AC ordirect current DC) between an electrode and the nanofiber sheet.Mechanical techniques include a razor blade configured to includesalient and reverse salient portions.

FIGS. 6A and 6B illustrate system configurations using two differenttypes of electrical discharge techniques in embodiments.

FIG. 6A schematically illustrates a single electrode system 600 forcontrolling a nanofiber sheet width using a corona or an arc generatedat a single electrode. In this example, the system 600 includes anelectrode 608 and, not shown, a power source connected to the electrode608. The nanofiber sheet 604 is shown in FIG. 6A to provide context forthe system 600. Electrical discharge 612 (whether the charged particlesof a corona or an arc between the electrode 608 and the nanofiber sheet604) is schematically illustrated emanating from the electrode 608. Asindicated above and as illustrated in FIG. 6A, the electrode 608 doesnot contact the nanofiber sheet 604. Rather, electrical discharge 612(whether a corona or an electrical arc) bridges a gap between theelectrode 608 and the nanofiber sheet 604, removing a portion of thenanofiber sheet 604 and thus dividing the nanofiber sheet 604 intosub-sheets.

The electrode 608 is connected to an electricity source and anelectrical controller (not shown). In one example, the electrode 608 isfabricated from a good electrical conductor having an electricalconductivity greater than 3×10⁷ Siemens/meter at 20° C., such as gold,copper, aluminum, among others. In another example, the electrode 608 isfabricated from tungsten. In some embodiments, the electrode is shapedwith a pointed tip. A pointed shape facilitates a high gradient in theelectric field generated at the electrode 608 relative to thesurrounding air. This in turn facilitates formation of a corona aroundthe pointed tip of the electrode or an electrical arc between theelectrode 608 and the nanofiber sheet 604 depending on the current,voltage, and a composition of an ambient atmosphere. It will beappreciated that a pointed tip is not required. In the example system600, the electrical controller connected to the electrode 608 can be anycontroller capable of producing a low current, high voltage conditionassociated with the electrode 608. Example currents can be within any ofthe following ranges: 0.1 milliAmp (mA) to 1 mA; 1 mA to 5 mA; 1 to 2mA; to 0.5 mA to 1 mA. Example voltages can be within any of thefollowing ranges: 50 Volts (V) to 22.0 V; 50 V to 110 V; 50 V to 75 V;75 V to 110 V. As indicated above, regardless of the value, the currentand voltage supplied to the electrode is consistent (e.g., within +/−5%)during the dividing of the nanofiber sheet 604 into sub-sheets.Consistent current and voltage improves consistency of the width ofmaterial removed, which would otherwise vary widely if the currentand/or voltage increased dramatically (or “spiked”) during processing.It will be appreciated that any variety of combinations of current andvoltage, even those not listed above, can be applied in accordance withthe present disclosure and that the preceding examples are provided fixillustration only.

One example type of a control system that can be used to provideelectrical current to the electrode 608 so that the electrode generatesa corona or an electrical arc is that of an electrical dischargemachining system (EDM) controller. Another example type of controlsystem is a pulse width modulation (PWM) controller. In some examples,the controller is used to maintain a consistent application of currentand/or voltage to the electrode 608. Generally, whether an EDMcontroller OF some other electrical controller is used to supply currentto the electrode 608, the electrical current and voltage supplied to theelectrode 608 are sufficient to form an electrical discharge 612 aroundthe electrode 608 consistent with the supplied voltage and current. As aresult of the electric field gradient around the electrode 608, atomsand/or molecules are removed from the nanofiber sheet 604 by theresulting electrical discharge, thus dividing the nanofiber sheet 604into sub-sheets, as described herein.

As shown, the nanofiber sheet 604 is disposed proximate to the electrode608. The electrode 608 is energized and the nanofiber sheet 604 is drawnso that atoms and/or molecules are removed from a portion of thenanofiber sheet 604 proximate to the electrode 608. In this way, theelectrode 608 (or multiple electrodes 608) can be used to divide thenanofiber sheet 604 into one or more sub-sheets, whether to confine adefect to a sub-sheet or to control the width of the sub-sheets for someother reason (e.g., to ultimately control diameter and variation indiameter of a nanofiber yarn fabricated from a sub-sheet). The nanofibersheet 604 is connected to an electrically grounded structure, whether agrounded growth substrate, yarn spinning assembly, or a bobbin used tocollect a nanofiber sub-sheet and/or spun nanofiber yarn.

In an example, a voltage between the electrode 608 and the nanofibersheet 604 can be increased to increase a dimension of the corona and/orelectric arc and thus increase a dimension of the corresponding portionof nanofibers removed from the nanofiber sheet 604. In other words, byincreasing this voltage, a gap between adjacent sub-sheets created fromthe nanofiber sheet 604 can be increased. In examples, this effect canbe used to control the width of a sub-sheet. In some examples, a 1 mAcurrent was applied to two electrodes 608 (spaced 1 millimeter (mm)apart) and the voltage was selectively varied to vary a width of thesub-sheet between the two electrodes 608. At 1 mA and 70 V applied tothe two electrodes 608, the sub-sheet between the electrodes had a widthof 900 microns (μm). At 1 mA and 80 V applied to the two electrodes 608,the sub-sheet between the electrodes had a width of 750 μm. At 1 mA and95 V applied to the two electrodes 608, the sub-sheet between theelectrodes had a width of 500 μm. In embodiments, the voltage andcurrent can be adjusted to produce sub-sheets as narrow as 5 μm. Thiscontrol allows for a feedback mechanism to take in-line yarn diametermeasurements and actively control the diameter of the CNT ribbons beingspun into yarns. Adjusting a width of a nanofiber sub-sheet by adjustingelectrode voltage is described below in more detail in the context ofFIG. 8.

In one example, a feedback system can measure yarn diameter directly orindirectly as the yarn exits a yarn spinner, and adjust a current and/orvoltage supplied to an electrode(s) (or even adjust a location of theelectrode relative to the sheet) to increase or decrease a width of thesub-sheet entering the yarn spinner. This adjustment can, in an example,help maintain a diameter of the nanofiber yarn being spun. In otherexamples, this feedback mechanism can be used to intentionally increaseor decrease a diameter of a portion of a nanofiber yarn relative to apreviously spun (or subsequently) spun portion of the nanofiber yarn.Examples of yarn diameter measurement system that can measure yarndiameter in situ include optical systems, such as a laser micrometer,and electrical systems (monitoring a conductance or resistance byplacing one or more probes in electrical contact with a segment ofnanofiber yarn), among others. For example, a density of a nanofibersub-sheet can decrease from a first value to a lower second value (dueto a tear defect or merely a low density of nanofibers within acorresponding area of the nanofiber forest). This can cause a diameterof nanofiber yarn spun from the nanofiber sheet to decrease from a firstdiameter to a smaller second diameter. To reduce the diametervariability (and variability in properties), at least one of a coronasize and an electrode location can be adjusted to increase the width ofthe nanofiber sub-sheet. By increasing the width of the sub-sheet, thequantity of nanofibers is increased so as to offset the decrease innanofiber density within the sub-sheet. This can restore the diameter ofthe nanofiber yarn to the first diameter, thus reducing diametervariability and property variability within the nanofiber yarn. Theresponsiveness of the system can reduce nanofiber yarn variation to lessthan +/−5% over sections as short as 1 mm. It will be appreciated thatthis type of feedback system is applicable to any of the embodimentsdescribed herein.

By way of illustration, in one example a nanofiber yarn diameter D canbe estimated and/or calculated according to Equation 1, where D is incentimeters (cm) W is nanofiber sheet width (cm), SD is nanofiber sheetareal density (grams/cm²), and ρ is nanofiber yarn density (g/cm³).

D={4*W*((SD/π)/ρ}^(1/2)  Equation 1

In examples, sheet areal density can be between 0.5×10⁻⁶ to 10×10⁻⁶g/cm² and yarn density (for yarns not including particles or infiltratedmaterials) can be from 0.1 to 1.6 g/cm³. In one specific exampleembodiment, a sheet selected to have a width of 20 mm and a sheet arealdensity of 2.8×10⁻⁶ g/cm² can produce a yarn having a density of 1.0g/cm³. A calculated diameter of this yarn is 26.7 microns (μm). It willbe appreciated that as the areal density of the sheet or the density ofthe yarn changes (as can be detected in situ during production),embodiments of the present disclosure can be used to adjust the sheetwidth to a value other than 20 mm so as to maintain a yarn diameter of26.7 μm or otherwise minimize the portion of yarn that deviates from. adiameter of 26.7 μm. In an analogous example, a sheet selected to have awidth of 1.1 mm and a sheet areal density of 2.4×10⁻⁶ g/cm² can producea yarn having a density of 0.9 g/cm³. A calculated diameter of this yarnis 6.1 microns (μm). As with the preceding example, embodiments of thepresent disclosure can be used to maintain the diameter of the yarnduring yarn production.

FIG. 6B illustrates a two-electrode arc discharge system 616 fordividing a nanofiber sheet 604 into sub-sheets. In this example,electrodes 620A and 620B are placed on opposite major surfaces of thenanofiber sheet 604. The electrical arc 624 between the electrode 620Aand the electrode 620B removes material from the nanofiber sheet 604.That is, when the nanofiber sheet 604 is drawn through the arc 624 (andnot in direct physical contact with electrodes 620A, 620B), the arc 624removes a portion of the nanofiber sheet 604, thereby dividing thenanofiber sheet 604 into sub-sheets, as described above. An advantage ofthe arc discharge system 616 is that the removal of the portion of thenanofiber sheet 604, and the size of the portion removed, is independentof the electrical conductivity of the nanofiber sheet 604. In otherwords, the electrical arc 624 forms between electrodes 620A and 620B,the size of which is not influenced by the electrical properties of thenanofiber sheet.

FIGS. 7A and 7B illustrate plan views of the system 600 used to divide ananofiber sheet into sub-sheets, in an embodiment. As shown in FIG. 7A,the view 700 includes a nanofiber forest 704, a substrate 708, and ananofiber sheet 720.

The nanofiber forest 704 and the substrate 708 have been described aboveand needs no further description. As shown, the nanofiber sheet 720 isdrawn from the nanofiber forest 704. A forest-sheet interface 716 movesin a direction opposite that of the direction in which the nanofibersheet 720 is drawn. Both of these directions are indicated in FIG. 7A.

As the nanofiber sheet 720 is drawn from the forest 704, theforest-sheet interface 716 moves toward defects 724A, 724B. The defects724A, 724B, and the potential effects on the integrity, consistency,density, and properties of the nanofiber sheet 720 have been describedabove and need no further description.

The view 700 also depicts coronas 726A, 726B, 726C, and 726D thatcorrespond to electrodes (not shown). It will be appreciated that inother embodiments, a one or two electrode arc discharge system andmethod can be applied. Regardless of whether a corona or an electricalarc is used, and regardless of whether one or two electrodes are used,the electrical discharge removes material of the nanofiber sheet 720,thus diving the nanofiber sheet 720 into nanofiber sub-sheets 728A,728B, 728C, 728D, and 728E.

Continuing with the scenario depicted in FIG. 7A, the view 730 in FIG.7B shows a state in which the nanofiber sheet 720 has been drawn so thatthe forest-sheet interface has progressed so as to pass beyond thedefects 724A and 724B, thus introducing defects 732A and 732B into thenanofiber sheet 720. However, in an embodiment of the presentdisclosure, the defects 732A and 732B are confined to sub-sheets 728Band 728D due to the dividing of the nanofiber sheet 720 into sub-sheets,as described above. In this way, sub-sheets 728A, 728C, and 728E have animproved consistency and uniformity, being free from defects. Asdescribed above, the defect-free and more consistent sub-sheets 728A,728C, and 728E can be used to produce one or more nanofiber yarns thathave a more uniform or consistent diameter (varying less than +/−5% overa 10 meter length of yarn), density (varying less than +/−5% over a 10meter length of yarn), and properties. These benefits are achievedwhether the individual sub-sheets 728A, 728C, and 728E are used toproduce corresponding separate nanofiber yarns or whether all of thesub-sheets 728A, 728C, and 728E are used to produce a single nanofiberyarn.

It will also be appreciated that locations of the electrodes 726A-726D(whether used to produce a corona or an electric arc) and the locationsof their corresponding electrical discharges (coronas in this case)relative to each other and relative to the nanofiber sheet can bechanged. In one example, the size of an arc or corona can be increasedor decreased by increasing or decreasing, respectively, a gradient of anelectric field generated by the electrode and/or a potential differencebetween electrodes, as described above. In another example, a mechanicalsystem can be used to physically move one or more of electrodes726A-726D. Regardless of the method used, changing a size and/or alocation of individual electrodes and their corresponding corona and/orarc enables widths of one or more sub-sheets to be controlled. Thiscontrol can be used to minimize a width of a sub-sheet that contains adefect, thus minimizing the loss of material associated with removing adefect. This control can also be used to maintain a mass of a sub-sheetbeing spun even as a density of the sub-sheet varies. Examples ofcontrol and feedback systems described above include laser micrometers,resistance/conductivity sensors, among others. Thus, even if thesub-sheet itself has a varying density of nanofibers per unit volume,the width of the nanofiber sub-sheet can be varied so that acorresponding nanofiber yarn spun from the sub-sheet is more consistent(e.g., in density, diameter, or in its properties).

FIG. 8 illustrates an embodiment in which sizes of a various electricaldischarges are individually selected so as to show an application ofthis parameter as used to control sub-sheet width around a defect, in anembodiment. The plan view 800 in FIG. 8 includes a nanofiber sheet 804,sub-sheets 806A, 806B, 806C, 806D, and 806E, a tear defect 808, andelectrical discharges 812, 816A, and 816B associated with correspondingelectrodes. As shown, a radius α of electrical discharge 812 issignificantly greater than a radius β of either of the electricaldischarges 816A or 816B.

Furthermore, electrical discharge size and electrode location can beused in cooperation with a defect, such as tear 808, to controlsub-sheet size. In the example shown, the electrodes corresponding toelectrical discharges 816A and 816B have been positioned approximatelyequal distances from corresponding edges of the tear 808. In cooperationwith the tear 808, these electrical discharges 816A and 816B dividesub-sheets 806C and 806D from the nanofiber sheet 804, thus takingadvantage of the presence of the tear 808 to create new sub-sheets. Theelectrical discharges 816A and 816B can be sized (using, voltage and/orcurrent supplied to the electrode) to fine tune a width of a sub-sheet.

FIGS. 9A and 9B illustrate a mechanical method for sub-dividing ananofiber sheet 912 into sub-sheets 916, 920, 924, and 928. In thisexample, a mechanical technique is used rather than the electricaltechniques described above. In this example, a commercially availablerazor blade, surgical blade, knife, or other similar blade is machinedto include at least one salient feature and at least one reverse salientfeature. In the example shown in FIG. 9A, a blade 900 is machined toinclude salient features 904A, 904B, and 904C, which are separated byreverse salient features 908A and 908B. These features can be machinedusing stamping, machine tools, EDM, among other techniques. A height δof the salient feature 904C (equally applicable to salient features 904Aand 904B) indicated in FIG. 9A can be any value greater than a thicknessof a nanofiber sheet. Example dimensions of the height δ are greaterthan 1 μm, greater than 20 μm, greater than 50 μm, or even greater than1 mm. A width ϵ of the salient feature 904C (equally applicable tosalient features 904A and 904B) indicated in FIG. 9A can be any value,from one micron to any value as wide as the nanofiber sheet to bedivided. The narrower the salient feature, the more likely the nanofibersheet is to be divided without loss of material. This conservesnanofiber sheet material that can then be spun into a nanofiber yarn, orotherwise used. Using the dividing device and method of FIG. 9 however,does not have the flexibility of the systems described above in thecontext of FIGS. 7 and 8 because the separation between salient featuresin the blade 900 is fixed.

FIG. 9B illustrates the effect on the nanofiber sheet 912 of the blade900. As shown in the plan view of FIG. 9B, the nanofiber sheet 912 hasbeen divided into sub-sheets 916, 920, 924, and 928, each of whichcorresponds to a reverse salient feature 908A, 908B or an area of thenanofiber sheet 912 not contacted by a salient features 904A, 904B,904C. Dividing the sub-sheets 916, 920, 924, and 928 occurs bycontacting the nanofiber sheet 912 with the blade edges of the salientfeatures 904A, 904B, 904C and drawing the nanofiber sheet 912 past thesalient features while maintain contact there between.

The above embodiments have a number of advantages, in addition toremoval of defects from a nanofiber sheet and the production of moreconsistent nanofiber yarns, as described above. For example, theembodiment described above can be applied to the production of nanofiberyarns that are less than 1 μm in diameter by sub-dividing a nanofibersheet into sub-sheets sheets that are from 3 μm to 7 μm wide. In someexamples, the sub-sheets have a width of 5 μm. Because nanofiber sheetsare delicate, sub-dividing nanofiber sheets into micron-scale sub-sheetsis difficult using conventional means.

Another advantage of embodiments described above is the capability tosimultaneously produce multiple nanofiber yarns. The nanofiber yarnsproduced, even sub-micron nanofiber yarns, can be of the same ordifferent diameters. This can be accomplished by merely applying one ormore embodiments described above and spinning each of the resultingsub-sheets.

FIG. 10 illustrates an example method 1000 for using at least oneelectrode to modify a width of a nanofiber sheet, which in some examplesis used to control uniformity of a property in a nanofiber yarn spunfrom the nanofiber sheet, as indicated above.

The method 1000 begins by providing 1004 a nanofiber sheet having afirst major surface and the second major surface. At least one electrodeis positioned 1008 proximate to, and not in direct contact with, atleast one major surface of the nanofiber sheet. A voltage is applied1012 at the least one electrode. In response to the applied 1012voltage, an electrical discharge is generated 1016 at the at least oneelectrode. In some examples as described above, the applied 1012 voltagegenerates 1017 a corona discharge. In some examples as described above,the applied 1012 voltage generates 1018 an electrical arc. Theelectrical arc can be between an electrode and the nanofiber sheet orbetween two electrodes positioned on opposite major surfaces of thenanofiber sheet and traveling through the nanofiber sheet. Regardless,the electrical discharge divides 1020 the nanofiber sheet into two ormore sub-sheets by removing a portion of the nanofiber sheet.

Optionally, an inhomogeneity can be identified 1024 in the nanofibersheet and the at least one electrode can be positioned proximate to theinhomogeneity so as to remove or exclude it from another portion of thenanofiber sheet. In some examples, a nanofiber yarn may optionally bespun 1028 from one of the sub-sheets. A yarn diameter or electricalproperties of the nanofiber yarn may be monitored 1032. In response tothe monitored yarn diameter or electrical property, the at least oneelectrode may be repositioned 1036 so as to maintain uniformity (asdescribed above at or within +/−5%) of the monitored property ordiameter.

Further Considerations

The foregoing description of the embodiments of the disclosure has beenpresented fix the purpose of illustration; it is not intended to beexhaustive or to limit the claims to the precise forms disclosed.Persons skilled in the relevant art can appreciate that manymodifications and variations are possible in light of the abovedisclosure.

The language used in the specification has been principally selected forreadability and instructional purposes, and it may not have beenselected to delineate or circumscribe the inventive subject matter. Itis therefore intended that the scope of the disclosure be limited not bythis detailed description, but rather by any claims that issue on anapplication based hereon. Accordingly, the disclosure of the embodimentsis intended to be illustrative, but not limiting, the scope of theinvention, which is set forth in the following claims.

What is claimed is:
 1. A method for dividing a nanofiber sheet, themethod comprising: configuring a structure to have at least one salientfeature comprising an edge, and at least one reverse salient feature;contacting the nanofiber sheet with an edge of the at least one salientfeature; and drawing the nanofiber sheet past the at least one salientfeature while maintaining contact therebetween, the drawing causing thenanofiber sheet to be dividing into sub-sheets.
 2. The method of claim1, wherein a width of the at least one salient feature is at least 1 μm.3. The method of claim 1, wherein a height of the at least one salientfeature is at least 1 μm.
 4. The method of claim 1, further comprisingidentifying an inhomogeneity in the nanofiber sheet, and whereindividing the nanofiber sheet comprises dividing the nanofiber sheet intoat least a first sub-sheet and a second sub-sheet, the inhomogeneitydisposed entirely within the first sub-sheet.
 5. The method of claim 4,wherein the inhomogeneity is a tear in the nanofiber sheet or avariation in a number of nanofibers per unit volume of the nanofibersheet.
 6. The method of claim 4, wherein at least one of the first orsecond sub-sheets is less than 5 μm in width.
 7. The method of claim 6,further comprising spinning a nanofiber less than 1 μm in diameter fromthe sub-sheet less than 5 μm in width.